It is required to, or at least expedient to, subject lens blanks made from phototropic glass compositions to a heat treatment, after initial forming thereof, to develop the phototropic characteristics or properties of the glass compositions from which the lens blanks are formed. Such a heat treatment is sometimes performed at a temperature above the softening point temperature of the glass of the lens blanks and, therefore, the lens blanks can sag or otherwise become misshaped if not suitably supported during said heat treatment thereof. For example, it may be required, or at least expedient to, subject lens blanks made from glass compositions such as disclosed in U.S. Pat. Nos. 3,197,296; 3,208,860, and 3,325,299, issued July 27, 1965, Sept. 28, 1965, and June 13, 1967, respectively, to heat treatments, at a temperature above the softening point temperature of the glass of the lens blanks, for the purpose of developing or inducing the phototropic characteristics or properties of said compositions and lens blanks made from such compositions. In such event said lens blanks should be suitably supported to prevent sagging or deformation thereof. The aforesaid supporting of said lens blanks has heretofore been performed by disposing and clamping the flat surfaces of each lens blank between the flat surfaces of a pair of flat disks of a suitable material and then subjecting such blank to the necessary heat treatment. This however imparts to said surfaces of the treated lens blank any roughness of said surfaces of said disks regardless of the minuteness of such roughness, and the treated lens blank, or an eyeglass lens formed, therefrom must, therefore, be ground and polished to remove said roughness imparted thereto during the heat treatment thereof.
It is economically desirable that the making of non-prescription sunglass lenses be performed at as low of a cost as possible and, to such end, it has been suggested that lens blanks can be sagged into suitably shaped mold carriers to form the contour or curvature desired for the sunglass lenses since such lenses are not prescription lenses requiring precise shaping by precise grinding thereof. It has then been further suggested that the lens blanks could be sagged into suitably shaped mold cavities to attain the desired eyeglass lens shapes during the aforesaid heat treatment and another economic advantage thereby attained. However, the surfaces of said mold cavities impart roughness to the convex surfaces of the shaped lenses in a manner similar to that in which roughness is imparted to the lens blanks by the surfaces of said disks during said heat treatment of such blanks and, therefore, said convex surfaces of the lens shaped by sagging of lens blanks into mold cavities would also have to be ground and polished to remove said roughness imparted thereto.
In view of the above it is an object of the present invention to provide a method of heat treating a plurality of lens blanks at a temperature above the softening point temperature of the glass of such blanks and without distortion of such lens blanks and, while such plurality of lens blanks are continued to be heated, to sag part of each of the blanks into a shape having a curvature desired for an eyeglass lens.
It is another object of the present invention to economically heat treat and sag lens blanks, made from a phototropic glass composition having a softening point temperature below the temperature required or desired for heat treatment of such blanks to develop the phototropic characteristics or properties of the lens blanks, into shapes having curved contours suitable for eyeglass lenses, the heat treatment and sagging of the lens blanks being performed on a plurality of said blanks at the same time.
Other objects and characteristic features will become apparent as the description proceeds.